The invention relates to a pulse resistor for a converter in the moderately high voltage and power range.
Converters having a DC voltage input are increasingly being used for regulated electrical drives and power supply installations in the moderately high voltage ranges. A converter of this type is also known as a voltage-source inverter. The standardized medium voltages 2.3 kV, 3.3 kV, 4.16 kV and 6.9 kV are counted as moderately high voltages.
FIG. 1 shows an equivalent circuit of a voltage-source inverter known from the prior art, of which just one load-side three-phase converter 2 is shown for reasons of clarity. Owing to the high voltage range, the converter valves T1-T6 of this load-side three-phase converter 2 each comprise a plurality of turn-off capable semiconductor switches 4 electrically connected in series, across each of which is connected a diode 6 in antiparallel. As each converter valve T1-T6 has three turn-off capable semiconductor switches 4, this converter topology is also called an on-off converter having a series connection number of Three. Every two converter valves T1,T2 and T3,T4 and T5,T6 respectively form a bridge path 8, which constitutes a phase module of the on-off converter 2. Each junction 10 between two converter valves T1,T2 or T3,T4 or T5,T6 forms a terminal L1 or L2 or L3 respectively for connecting a three-phase load, for example a three-phase motor. The three phase modules 8 of the three-phase converter 2 are electrically connected in parallel by two busbars P0 and N0. A DC-link circuit capacitor CZW is connected between these two busbars P0 and N0, said capacitor comprising, for example, one or a plurality of capacitors electrically connected in series and/or parallel. A DC voltage Ud lies across this DC-link circuit capacitor CZW. In this equivalent circuit of an on-off converter having a series connection number of Three, insulated gate bipolar transistors (IGBT) are provided as the turn-off capable semiconductor switches 4. The series connection number depends on the DC voltage Ud lying across DC-link circuit capacitor CZW and on the blocking ability of commercially available IGBTs.
With temporary energy recovery in the DC-link circuit capacitor CZW, the DC voltage Ud lying across the DC-link circuit capacitor CZW can increase such that it exceeds a maximum permissible value for this DC voltage. Such a situation occurs in particular during braking of a three-phase motor connected to the terminals L1, L2 and L3. Other causes that are generally of short duration, such as rapid fluctuations of the line voltage of a grid supply or load fluctuations, can also produce such overvoltages. The following measures are known for overcoming these problems:                Connecting a converter with an energy-recovery facility, the converter being electrically connected in parallel with the DC-link circuit capacitor CZW. The excess energy from the DC-link circuit capacitor CZW can thereby be fed back into a grid system that is able to receive power.        Connecting a pulse-controlled resistor across the busbars P0,N0 of the DC-link circuit, said resistor being used to convert the excess energy of the DC-link circuit capacitor CZW into heat.        
FIG. 2 shows an equivalent circuit of a pulse-controlled resistor, also known as a pulse resistor. This known pulse resistor comprises a final control element 12 and a resistance element 14. A phase module 8 is used as the final control element 12, for which the turn-off capable semiconductor switches 4 of the lower converter valve T8 are not needed. The implementation of the upper converter valve T7 of this phase module 8 is the same as the implementation of the converter valve T1 or T3 or T5 respectively of the load-side three-phase converter 2 shown in FIG. 1. To aid understanding, the turn-off capable semiconductor switches 4 of the lower converter valve T8 of the final control element 12 of the pulse resistor are not shown explicitly in the equivalent circuit diagram. These can, however, be present in the phase module 8, but are not actuated with the “brake” function. The resistance element 14 is electrically connected in parallel with the lower converter valve T8 having the series connection number of Three. This resistance element 14 comprises a resistive and an inductive component 16 and 18. The inductive component 18 represents its parasitic inductance. This pulse resistor has the following disadvantages for high voltages:    a) The currents iP and iN in the supply lines 20 and 22 of the pulse resistor have a very high rate of current rise di/dt, resulting in emission of electromagnetic interference.    b) The supply lines 20 and 22 must be made physically short and of low inductance in order to limit the voltages arising across the turn-off capable semiconductor switches 4.    c) This pulse resistor has an on-off response and in the periodic pulsed operation generates a high AC component of the current iP and iN in the supply lines 20 and 22.    d) In order to perform its function, this pulse resistor requires a DC capacitor CZW to be physically located as close as possible, i.e. this pulse resistor must be physically positioned immediately beside the DC-link circuit capacitor CZW.
The disadvantages of points a) and b) are particularly troublesome if the pulse-controlled resistor 14 is to be used as an optional add-on to the converter 2. The disadvantage stated in point c) results in increased ripple on the DC voltage Ud of the DC-link circuit capacitor CZW of the one-off converter 2 having the series connection number of Three. This increased ripple has unwanted repercussions for the operation of other converters connected to the busbars P0, N0. The disadvantage stated in point d) means that this pulse resistor cannot be used with converter topologies that do not comprise a DC-link circuit capacitor CZW.